OBSERVATION OF THE SINGLET SINGLET CRYSTAL-FIELD EXCITONS IN PRCU2 BYINELASTIC NEUTRON-SCATTERING

The crystal-field transitions in the paramagnetic state of PrCu2 were studied by means of inelastic neutron scattering between 1.8 and 10 K in an energy transfer range 0.5-2.5 meV. We observed excitations from the singlet ground state to two singlet excited states. They were assi gned as a J(x)-polarized and a J(y)-polarized transition. The J(x) tra nsition has acoustic and optical branches. While the J(y) transition i s very weakly dependent on the wavenumber, the J(x) transitions are st rongly dispersive, indicating an anisotropic RKKY interaction between the 4f electrons. A substantial softening of the exciton energy was ob served in the optical branch of the J(x) transition as a sharp minimum at the wavevector Q(m) = 0.24a + 0.68c*, which is the modulation vec tor of the magnetic order below T-N = 54 mK. The manner of softening a bove 1.8 K is discussed in terms of the mean-field theory.